1. Field
The present invention relates generally to data communication, and more specifically to techniques for detecting and mitigating adjacent channel interference (ACI) in a wireless (e.g., CDMA) communication system.
2. Background
Wireless communication systems are widely deployed to provide various types of communication such as voice, data, and so on. These systems may be multiple-access systems capable of supporting communication with multiple users and may be based on code division multiple access (CDMA), time division multiple access (TDMA), frequency division multiple access (FDMA), or some other multiple access techniques. CDMA systems may provide certain advantages over other types of system, including increased system capacity.
A CDMA system may also be designed to implement known CDMA standards such as IS-95, cdma2000, IS-856, W-CDMA, other standards, and variants thereof. Each CDMA standard typically specifies a particular chip rate for spreading data prior to transmission and a particular minimum spacing between carrier signals on the forward link. The chip rate determines the bandwidth of a modulated signal transmitted on a CDMA channel, and the carrier frequency determines the center frequency of the modulated signal. For example, in IS-95A/B, cdma2000-1X, and cdma2000-1X EV, the chip rate is defined as 1.2288 Mcps and the minimum spacing between carrier signals is 1.23 MHz for cellular band (i.e., 800 MHz range) and 1.25 MHz for PCS band (i.e., 1900 MHz range).
Each CDMA standard typically further specifies a particular transmit mask for the modulated signal. The transmit mask defines the maximum and minimum amplitude values for the modulated signal over a range of frequencies. The transmit mask thus effectively band-limits each CDMA channel's baseband transmit pulse to less than half of the chip rate (i.e., |f|<W/2, where W is related to the chip rate and is equal to 1.2288 MHz for IS-95 and cdma2000-1X). Since the transmit mask specification is not stringent, each CDMA channel “bleeds” some amounts of signal energy into adjacent channels where it is perceived as adjacent channel interference (ACI).
ACI is one of the components that make up the total noise and interference observed at a receiver (e.g., a terminal). It can be shown that ACI increases the noise floor by limiting the maximum signal-to-noise-and-interference ratio (SINR) to approximately 13 dB and 15 dB for channel spacing of 1.23 MHz and 1.25 MHz, respectively, on the forward link.
In a CDMA system, the data for each user is spread over the entire system bandwidth. The spreading provides processing gain, with the amount of gain being dependent on the ratio of the system bandwidth to the data rate. When the data rate is low (e.g., for voice and low rate data), the processing gain is large and this allows the data to be recovered at a low SINR. For example, in IS-95A/B and cdma2000-1X systems, the terminals typically operate at an SINR of 0 dB or less. At such low SINR levels, the thermal noise and the interference due to transmissions from other transmitters dominate, and the contribution due to ACI is typically negligible in comparison to these other components.
However, as the data rate increases, the processing gain decreases and a higher SINR is needed to achieve the desired level of performance. For example, in a cdma2000-1X EV system, a SINR of approximately 10 dB is needed for the highest data rates supported on the forward link. ACI is then a non-negligible component that may have a large impact on the overall SINR.
There is therefore a need in the art for mitigating ACI in a CDMA system to achieve higher SINR levels needed for higher data rates.